Filtration occurs by passing a contaminated medium, such as a fluid or gas, through a barrier material, called a filter membrane, that removes most, but not necessarily all contaminates in the medium. Typically, the contaminates are particles in a concentration that can vary from a few parts per million to such a high concentration that the medium will not easily flow through the filter membrane. The contaminates, however, need not always be particles but may also be fluid or gases of a specific gravity different from the medium being filtered.
Sparging, a type of filtration, occurs when the filter membrane is used to separate a gas from a fluid stream in such a way that the separated gas is allowed to dissipate into the atmosphere or is carried away from the membrane for use or disposal. Sparging also occurs when the filter membrane is used to introduce a gas into a fluid stream at a desired rate.
For any given filter membrane, the exact percentage of particulate contamination the filter membrane will retain during filtration will vary with the particle size of the particulate contamination in the medium being filtered and the micron rating of a filter membrane. Furthermore, as contaminates are trapped on the inlet surface of the filter membrane or within the depth of the filter membrane, the filtration characteristics of the filter membrane will change. As filtration proceeds, resistance to flow of the medium being filtered increases because of the ever increasing presence of contaminates trapped by the filter membrane. As the resistance to flow increases, however, the efficiency of the filter membrane improves because trapped particulate contaminants trap other contaminants which might not have otherwise been trapped by the filter membrane. At some point in the life of the filter membrane, however, resistance to flow through the filter membrane becomes so high that the filter membrane must be replaced or, if possible, backflushed to remove all contamination.
Other characteristics of a filter membrane are permeability, retention rate and capacity, and each have a profound effect upon the performance and cost of the filter membrane. Permeability, which should not be confused with porosity, is the rate at which a given medium will flow through a filter membrane under given conditions. It is the inverse of resistance to flow. Porosity, however, is the fraction of air void within the filter membrane.
Retention rate, also referred as efficiency, is the ability of a given filter membrane to remove particulate contamination of a given size. The retention rate of a filter membrane is indicated by its micron rating, which is determined by a non-destructive "ethynol bubble point" test.
The capacity of a filter membrane is its ability to filter over time before its permeability drops below a given level. For example, many filter membranes are evaluated by passing a fluid through the membrane at a constant rate of flow while gradually introducing particulate contamination and measuring the increase in pressure drop across the membrane over time. The capacity of the filter membrane, and therefore its useful life, is indicated when the pressure drop, and therefore the permeability of the filter membrane, reaches a predetermined level.
As can be surmised, a correlation between permeability and retention rate exists. The higher the retention rate of a given filter medium, the lower the permeability of the filter medium. Thus, if one were able to adjust the porosity, and therefore adjust the retention rate, of a filter membrane during conventional filtration, one could vary the permeability of the filter membrane during filtration or one could maintain a constant permeability for the filter membrane as ever increasing amounts of particulate contamination are trapped by the membrane.